Around 1988-1989, Wei Yen asked Rikk Carey to lead the IRIS Inventor project. Their goal was to create a toolkit that made developing 3D graphics applications easier to do. The strategy was based on the premise that people weren't developing enough 3D applications with OpenGL because it was too complicated, and that if 3D programming were made easier then more people would create 3D applications and SGI would benefit. Therefore, the credo was always "Ease of Use" before "Performance", and soon the tagline "3D Programming for Humans" was being used widely.
OpenGL (OGL) is a low level library that takes lists of simple polygons and renders them as quickly as possible. To do something more practical like "draw a house", the programmer must break down the object into a series of simple OGL instructions and send them into the engine for rendering. One problem is that OGL preformance is highly sensitive to the way these instructions are sent into the system, requiring the user to "know" which instructions to send and in which order, and forcing them to carefully "cull" the data to avoid sending in objects that aren't even visible in the resulting image. For simple programs a tremendous amount of programming has to be done just to get started.
Open Inventor (OI) was written to address this issue, and provide a common base layer to start working with. Objects could be subclassed from a number of pre-rolled shapes like cubes and polygons, and then easily modified into new shapes. The "world" to be drawn was placed in a scene graph run by OI, with the system applying occlusion culling on objects in the graph automatically. OI also included a number of controller objects and systems for applying them to the scene, making common interaction tasks easier. Finally, OI also supplied a common file format for storing "worlds", and the code to automatically save or load a world from these files. Basic 3D applications could then be written in a few hundred lines under OI, by tying together portions of the toolkit with "glue" code.
On the downside OI tended to be slower than hand-written code, as 3D tasks are notoriously difficult to make perform well without shuffling the data in the scene graph by hand. Another practical problem was that OI could only be used with its own file format, forcing developers to write convertors to and from the internal system.
Open Inventor was later opened for 3rd-party licensing, which is when it switched from "IRIS" to "Open". It was licensed to two third party developers, Template Graphics Systems and Portable Graphics. TSG later bought Portable Graphics, making them the sole licensee.
About a year into the Inventor project, a different philosophy began to emerge. Instead of simply making it easy to write applications on SGI systems, now the goal was to make it difficult to write slow applications. Members of the Inventor team left to form their own group, forming the basis of the OpenGL Performer project (then still known as IRIS Performer). Performer was also based on an internal scene graph, but was allowed to modify it for better speed as it saw fit, even dropping "less important" objects and polys in order to maintain guaranteed performance levels. Performer also used a number of threads to run tasks in parallel for added performance, allowing it to be run (in one version) on multiple processors. Unlike Inventor, Performer remained proprietary so that SGI would have the agility to modify the API as needed to keep in step with the latest hardware enhancements.
At some point in the mid-1990s it was realized that there was no good reason that the two systems could not be combined, resulting in a single high-level API with both performance and programmability. SGI started work on yet another project aimed at merging the two, eventually culminating in Cosmo 3D. However Cosmo had a number of practical problems that could have been avoided with better design.
Eventually all of these ideas would come together to create the OpenGL++ effort, along with Intel, IBM and DEC. Essentially a cleaned up and more "open" version of Cosmo 3D, work on Cosmo ended and SGI turned to OpenGL++ full time. The OpenGL++ effort would drag on and eventually be killed, and SGI then tried again with Microsoft with the similar Fahrenheit project, which also died. During this time SGI ignored OI, and eventually spun it off completely to TGS.
After many years of Inventor being ignored, it was released in open source in August 2000, and is now available on SGI's web site. However today OI is no longer a competitive system, and appears to have seen little use. None of the source for SGI's later efforts such as OpenGL++ has ever been released.